The intrusion detection apparatus comprises two principal components, an electrical cable of unique design and an instrument commonly known as a time-domain reflectometer (TDR), which, when suitably combined, provide an electrical response to, and a video display of, induced cable motion or vibration. This motion or vibration may occur at a single point, at any number of points, along a continuous portion of the cable, or along the entire length of the cable. The horizontal trace on the video display represents the length of the cable and the occurrence of cable motion or vibration is displayed as a vertical displacement of a trace at a point or region whose location within the trace corresponds to the location of the actual motion or vibration within the length of the cable. Thus, this invention provides an indication of both the occurrence and location of motion or vibration within the length of the cable.
The process of monitoring motion and vibration over large distributed areas is accomplished in the prior art with two classes of devices: point sensors and short line sensors which provide no location resolution. The short line sensors, which may typically be up to 100 meters in length, function as a single sensor in that an indicated response cannot be identified with a particular point along the line. None make use of the time-domain reflectometry cable technology which is the subject of this invention. Thus, as in the case in most if not all applications, many of these short line sensors must be used in order to obtain the required area monitoring, and each line sensor requires a separate link monitoring room.
Point sensors such as microphones, geophones, and accelerometers are used to provide monitoring of specific points, and are placed in a continuous line in order to accomplish the monitoring of property boundaries. In the latter instance, very large numbers of sensors are required, and each must have a separate link to the central monitoring rooms.
In most of the common applications of existing line point sensors, some source of electrical power is required at the sensor end of the link to operate the link circuits. Thus, either batteries must be used and periodically charged, or a separate power supply line must be incorporated in the system.
Because of the great numbers of sensors which are generally required, central station data processing is generally simplified through the application of threshold circuits at the sensor end of the link. In this case, only one signal level is transmitted and this occurs when the sensor signal exceeds the threshold minimum. Thus, no magnitude or frequency analysis is possible, as in the apparatus of this invention.